Palaeovegetational reconstruction in Late Miocene: A case study based on early diagenetic carbonate cement from the Indian Siwalik

Abstract

Carbon isotope ratio of early diagenetic carbonate cement (EDCC) from sandstones was measured from Mohand Rao (age: 9 to 4.5 Ma) and Haripur Khol (6 to 1.8 Ma) sections of Siwalik Group of India to reconstruct palaeovegetation. The δ 13C of cement from Mohand Rao section varies from − 10.5‰ to − 0.2‰ with progressive increase in values from 9 to 7.3 Ma indicating gradual change of existing C 3 type vegetation to C 4 type vegetation. Post 7.3 Ma, the δ 13C value is anchored around zero per mil indicating mixed C 3–C 4 environment with C 4 dominating the ecosystem. In Haripur Khol section, the δ 13C value of EDCC from sandstone indicates presence of both C 3 and C 4 type of plants with dominance of C 4 in the ecosystem, which corroborates the results of our previous study based on carbon isotope ratio of soil carbonates. The oxygen isotope ratio of EDCC from sandstones does not show any systematic variation with time. The δ 18O values of EDCC from Mohand Rao section ranges from − 8.9‰ to − 13.6‰ and in Haripur Khol section, from − 9.9‰ to − 13.6‰. At a given stratigraphic level, the average δ 18O value of EDCC of sandstones is lower (up to 4‰) compared to the average δ 18O of soil carbonate from the same or adjacent level (using our earlier published data from Haripur Khol section). The depletion in 18O of EDCC may be due to contribution of contemporary river water infiltrating the groundwater system in the post-monsoon period. Carbon and oxygen isotope ratio of soil carbonate nodules and carbon isotope ratio of associated organic matter from the same nodules were also measured from the Mohand Rao section in a few cases ( n = 9). From 9 to 8 Ma, the carbon isotope ratio of soil carbonates varies from − 10.8‰ to − 7.8‰ indicating dominance of C 3 type vegetation in the flood plain. In contrast, from 5.4 to 4.8 Ma, the δ 13C ranges from 0.1‰ to − 4.3‰ indicating that the vegetation consisted of mixed C 3–C 4 plants with C 4 dominating the ecosystem. The carbon isotope ratio of the organic matter from the same soil carbonate nodules ranges from − 25.2‰ to − 24.4‰ (from 9 to 8 Ma) and − 17.4‰ to − 24.6‰ (from 5.4 to 4.8 Ma) corroborating the above results. The average δ 18O value of soil carbonate nodules for the time period 9 to 8 Ma is − 8.8‰; for 5.4 to 4.8 Ma, the value is − 7.9‰. These average δ 18O values are comparable with previously published oxygen isotope results of soil carbonates from Haripur Khol and Kangra valley.